1. Field of the Invention
The present invention relates to a powder feeding apparatus, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method. More specifically, the present invention relates to a powder feeding apparatus for feeding a powder into a cavity formed in a die, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method.
2. Description of the Related Art
Currently, as sintered rear-earth alloy magnets, two kinds, i.e. a samarium-cobalt magnet and a rare-earth-iron-boron magnet, are used extensively in many fields. Of the two, the rare-earth-iron-boron magnet is appreciated in application to variety of electronic devices and apparatuses. (Hereinafter, the rare-earth-iron-boron magnet will be called xe2x80x9cR-T-(M)-B magnetxe2x80x9d, where R represents a rare-earth element including yttrium, T represents iron or iron partially substituted by a transition metal element, M represents a doped element, and B represents boron.) A reason for this is that the R-T-(M)-B magnet is the most superior of many kinds of magnets in terms of magnetic energy product and relatively inexpensive in terms of price. The transition metal included as T may be cobalt for example. Boron can be partially substituted by carbon.
In manufacture of such a rare-earth magnet, first, a magnetic alloy powder made by milling a rare-earth alloy is pressed into a compact (green compact) by a pressing apparatus. When making the compact, the magnetic alloy powder is fed into a cavity formed by a die hole (through hole) provided in a die and a lower punch inserted into the die. The magnetic alloy powder fed in the cavity is pressed by an upper punch. The compact thus obtained is then sintered at a temperature of 1000xc2x0 C. -1100xc2x0 C. approx., and then finished as the sintered rare-earth magnet.
Conventionally, a variety of methods are proposed for feeding the magnetic alloy powder into the cavity in the pressing apparatus.
For example, Japanese Utility Model Publication (of examined Application for opposition) No. 59-32568 and Japanese Patent Laid-Open No. 61-147802 each discloses a technique of vibrating a container which holds the powder and thereby supplying the power into the cavity in sieving action through a metal net.
According to Japanese Patent Laid-Open No. 61-147802, there is described an apparatus comprising a feeder cup (the powder container) having a bottom portion provided with a metal net. The feeder cup is vibrated relatively rigorously by using a solenoid coil, thereby feeding the granular magnetic powder through the metal net into the cavity in a short time.
However, according to the conventional apparatus disclosed in Japanese Patent Laid-Open No. 61-147802, the vibration is generated by means of attracting force between the solenoid coil and an iron core, and of restoring force provided by a spring, and the vibration is given to the feeder cup itself which holds the powder. The iron core (moving part) is fastened to the feeder cup by a connecting hardware. With this arrangement, the vibrating force transmitted to the powder in the feeder cup is only a reciprocating force, and the transmitted force is still not sufficient to break down a lump of powder. In such an apparatus, in order to supply the granular powder into the cavity while preventing bridge formation, one possibility is to use the metal net having a fine grid (mesh). However, use of such a fine-mesh metal net poses another problem that the powder is not quickly sieved and there is a significant increase in the time for feeding the powder.
Another problem with the above conventional apparatus is that it is difficult to increase the stroke (amplitude) of vibration given to the feeder cup. If the feeder cup is moved only in a short stroke, it is difficult to feed the powder uniformly in the cavity.
There is still another problem. Specifically, corner and/or edge regions of the cavity is more difficult to feed with the powder than a center region of the cavity. According to the conventional apparatus therefore, when the rare-earth alloy powder is supplied through the metal net which is provided at a position relatively high above the die surface, the powder tends to form a high portion in the center region. If the powder is fed in such a non-uniform density in the cavity, the compact formed by the pressing operation has an unacceptably large difference in its pressing density, between the corner and/or edge regions and the center region. This density difference can cause a crack in the compact.
This problem is presumable also in an apparatus disclosed in Japanese Utility Model Publication (of examined Application for opposition) No. 59-32568.
Other techniques for feeding the powder into the cavity are proposed in Japanese Patent Laid-Open No. 11-49101 and Japanese Patent Laid-Open No. 2000-248301.
According to the technique disclosed in Japanese Patent Laid-Open No. 11-49101, a feed is fed into a container by means of pneumatic tapping and via a supplying hopper. An arrangement is made so that the feed is present in both of the supplying hopper and the container after the pneumatic tapping. Then, of this mass of the feed present in both of the supplying hopper and the container, a portion of uniform density formed in the container is separated from the feed remaining in the supplying hopper.
Japanese Patent Laid-Open No. 2000-248301 discloses a supplying apparatus, in which a feeder box having an opening in a bottom is moved to above a cavity formed in a die tooling, allowing a rare-earth alloy powder to be supplied into the cavity from the opening. The supplying apparatus comprises rod members which are moved at the bottom portion horizontally within the feeder box. The rod members are reciprocated when the rare-earth alloy powder in the feeder box is supplied to the cavity.
However, according to the technique disclosed in Japanese Patent Laid-Open No. 11-49101, since the feeding into the container is performed by the pneumatic tapping, the feeding density of the feed in the container becomes higher than by means of natural gravitational fall. For example, a rare-earth alloy powder fed by means of natural gravitational fall has the feeding density of 1.8 g/cm3 approx., versus the feeding density of 3.4 g/cm3 approx. by means of pneumatic tapping. The feed packed to such a high density does not allow particles of the powder to move easily, requiring a stronger magnetic field in order to orient the powder, leading to increase in manufacturing cost.
According to the technique disclosed in Japanese Patent Laid-Open No. 2000-248301 on the other hand, as shown in FIG. 21A, a feeder box 2 is moved toward a cavity 1. Then, as shown in FIG. 21B, when the feeder box 2 is positioned above the cavity 1, a powder 3 is supplied into the cavity 1 by the weight of the powder 3 itself. The feeding thus performed is not even, and therefore the powder 3 is not distributed uniformly. Thereafter, as shown in FIG. 21C and FIG. 21D, a shaker 4 is activated to fill the cavity 1 with the powder 3. The shaker 4 forces the powder 3 in, to the density of 2.3 g/cm3 approx., thereby uniformalizing the feeding density. As a result, a stronger magnetic field is necessary in order to obtain a desired level of orientation. FIG. 22 shows state changes in the feeding operation performed by this conventional apparatus.
Further, if the cavity is shallow in a direction of the pressing operation provided by the punches, the feeding density inconsistency in the cavity is not easily corrected by the pressing operation, leading to occasional crack development in the compact.
It is therefore a primary object of the present invention to provide a powder feeding apparatus, a pressing apparatus using the same and a sintered magnet manufacturing method, capable of feeding the powder uniformly and in a short time into the cavity of the pressing apparatus.
Another object of the present invention is to provide a powder feeding apparatus, a pressing apparatus using the same, a powder feeding method and a sintered magnet manufacturing method, capable of providing a desired orientation and a high magnetic characteristic at a low cost.
According to an aspect of the present invention, there is provided a powder feeding apparatus for feeding a powder into a cavity formed in a die, comprising: a container including a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing the powder to pass through; and an impactor capable of hitting against the container; wherein the impactor is hit against the container to give an impulsive force to the container, thereby feeding the powder contained in the container into the cavity via the openings.
According to this invention, by having the impactor hit against the container, a lump of the powder contained in the container can be broken down and the powder in the broken state can be supplied into the cavity.
According to another aspect of the present invention, there is provided a pressing apparatus comprising: the above described powder feeding apparatus; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus.
According to still another aspect of the present invention, there is provided a sintered magnet manufacturing method comprising: a first step of applying an impulsive force to a container which includes a bottom portion provided with a powder holding portion formed with a plurality of openings capable of allowing the powder to pass through, thereby feeding the powder contained in the container via the openings into a cavity formed in a die; a second step of forming a compact by pressing the powder fed in the cavity; and a third step of manufacturing a sintered magnet by sintering the compact.
By pressing the powder which is fed uniformly in the cavity, a compact which has a uniform density, and a small inconsistency in size and weight can be manufactured.
Further, by sintering the compact, a magnet which has a small inconsistency in size and weight can be obtained.
Preferably, the apparatus further comprises a vibrating mechanism connected to an upper portion of the container. The impactor is provided so as to hit against a lower portion of the container, and the vibrating mechanism vibrates an upper portion of the container, thereby allowing the impactor to hit against the lower portion of the container. In this way, by connecting the vibration mechanism with the container and by separating the impactor from the vibration mechanism, it becomes possible to reduce whirling up of the powder, thereby reducing binding of the powder in the vibrating mechanism. Further, by hitting the impactor on the lower portion of the container, the impact can be transmitted more directly to the opening of the container, making possible to transmit the impact to the entire mass of the powder present at the opening, thereby feeding the cavity with the powder uniformly.
Further, preferably, the powder holding portion is formed of a net having a mesh size of 2-14. More preferably, the powder holding portion is formed of a net having a mesh size of 2-8. By using a relatively coarse net as the above, the powder can be fed uniformly into the cavity while remarkably reducing the time necessary for the powder feeding.
Preferably, the powder holding portion is provided at a height smaller than 2.0 mm from a surface of the die. More preferably, the powder holding portion is provided at a height smaller than 1.0 mm from the surface of the die. This arrangement makes possible to allow only a small amount of the powder to project from within the cavity above the surface of the die. Therefore, an amount of the extra powder to be wiped is small, and a lump produced in the wiping operation by the container is not unwontedly fed into the cavity at the next cycle of powder feeding.
Further, preferably, the container can move when the impulsive force is given to the container by the hitting of the impactor against the container. With this arrangement, it becomes possible to have the moving container be hit by the impactor, and to give a reverse impact to the container, and therefore to feed the cavity with the powder more uniformly.
Preferably, the apparatus comprises a plurality of the impactors disposed outside of the container in an opposing relationship, with the container in between. With this arrangement, the impulsive force can be given continuously to the container.
Further, preferably, the apparatus further comprises a partition plate provided inside the container. With this arrangement, when the impactor hits a side wall of the container, the impulsive force can be transmitted dispersively to the powder inside the partitioned container, making possible to feed the powder more efficiently. This arrangement can remarkably reduce feeding time of the powder into the cavity.
Further, preferably, a size of the openings provided in the powder holding portion is in accordance with a location of the opening. By changing the coarseness according to the location of the opening in this way, the amount of powder to be fed into the cavities can be controlled according to region.
If the powder is a rare-earth alloy powder, the powder particles are angular, and with addition of a lubricant, the powder decreases its flowability and forms a lump, into a state not to easily drop from the opening of the powder holding portion. However, according to the present invention, even if the powder is a rare-earth alloy powder mixed with a lubricant and poor in flowability, the powder can be fed in the cavity uniformly and efficiently in a short time.
According to another aspect of the present invention, there is provided a powder feeding apparatus for feeding a powder into a cavity formed in a die, comprising: a feeder box movable to above the cavity, including a bottom portion formed with an opening, and containing the powder; a rod member provided inside the feeder box and pushing the powder downwardly; a linear member provided at the opening of the feeder box; and orienting means which aligns the powder fed from the feeder box in the cavity.
According to still another aspect of the present invention, there is provided a powder feeding method for feeding a powder into a cavity formed in a die, the method comprising: a step of moving a feeder box to above the cavity of the die, with the feeder box containing the powder, being provided inside thereof with a rod member movable in a horizontal direction, and having an opening provided with a linear member; a step of feeding the powder into the cavity while moving the rod member in the horizontal direction within the feeder box, when the feeder box is above the cavity; and a step of orienting the powder by applying a magnetic field to the powder in the cavity.
According to this invention, by providing the linear member at the opening of the feeder box, the powder does not fall into the cavity even when the feeder box has moved to above the cavity. The powder can be fed into the cavity thereafter, by activating the rod member in the feeder box. In this feeding, the powder can be fed into the cavity uniformly at a natural feeding density (1.7 g/cm3-2.0 g/cm3 for example). Since the powder is not fed at a high density, the powder particles can move easily, and a desired orientation can be achieved by an orienting magnetic field of a relatively low strength. This makes possible to prevent manufacturing cost from increasing. Further, since the density distribution in the feeding can be made uniformly, a product having a superb magnetic characteristic can be obtained by orienting the powder in the cavity.
Preferably, the rod member is spaced from the linear member by a distance not smaller than 0.5 mm and not greater than 10 mm. With this arrangement, flow of the powder near the linear member is assisted, making possible to smoothly feed the powder into the cavity at a density suitable for the orientation.
According to still anther aspect of the present invention, there is provided a pressing apparatus comprising: the powder feeding apparatus described above; and pressing means which presses the powder fed in the cavity by the powder feeding apparatus.
According to this invention, by pressing the powder which is fed in the cavity by the above powder feeding apparatus, a compact high in density uniformity can be obtained, and thus crack and fracture development due to inconsistent density can be prevented.
If the powder is produced by using a rapid quenching process, and a particle distribution pattern of the powder is made narrow, the powder has an extremely poor flow ability. However, according to the present invention, since the powder flowablity can be improved by the natural gravitational feeding, density consistency of the powder in the cavity can be improved even if the powder is produced by using the rapid quenching process and the particle distribution pattern of the powder is made sharp. Further, each powder particle can be easily moved, and therefore it becomes possible to form a magnet having a high magnetic anisotropy for example.
Preferably, the interval between the linear members is not smaller than 2 mm and not greater than 12 mm.
According to still anther aspect of the present invention, there is provided a sintered magnet manufacturing method comprising: a step of obtaining a compact by pressing a powder in a cavity, the powder being fed by the above described powder feeding method; and a step of manufacturing a sintered magnet by sintering the compact.
According to this invention, by pressing the powder fed into the cavity by means of the above described method, a compact high in density uniformity can be obtained, and thus crack and fracture development in the compact can be reduced. As a result, sintered magnet obtained by sintering the compact has a decreased rate of defects due to cracking and/or fracturing, and a decreased rate of deformation. Therefore, it becomes possible to improve yield in manufacturing process, to improve productivity of the sintered magnet, and to manufacture a sintered magnet having a favorable magnetic characteristic.
The above objects, other objects, characteristics, aspects and advantages of the present invention will become clearer from the following description of embodiments to be presented with reference to the accompanying drawings.